In the prior art, it is known to produce aluminum in coil form from a continuous casting apparatus wherein molten aluminum is delivered from a tundish and cast in the form of a metal sheet or strip and wound on a coiler. Generally, in this process, molten aluminum is deposited on a moving chill surface from a tundish having an open outlet. An inlet is provided for the flow of molten metal into the tundish from a source of molten metal. The direct casting of the molten aluminum metal onto a chill wheel, preferably a grooved chill wheel, produces a cast aluminum product at a rapid rate. The aluminum cast strip is wound on a coiler in heated form, generally at a temperature in the range of about 400.degree.-1000.degree. F.
Drag casting apparatus and methods of this type are described, for example, in U.S. Pat. Nos. 4,828,012, 4,896,715, 4,934,443, 4,934,443, 4,945,974, 4,940,077 and 4,955,429. The disclosures of these patents are hereby specifically incorporated by reference with respect to the method and apparatus for the production of aluminum strip and coil from molten aluminum or aluminum alloys.
With reference now to FIG. 1, a continuous casting apparatus is illustrated which is typical of prior art continuous casting apparatus using a driven chill surface. The continuous casting apparatus is generally designated by the reference numeral 10 and is seen to include a tundish 1 positioned adjacent a driven chill surface 3. The chill surface 3 comprises the external cylindrical surface of a casting wheel 5. The casting wheel 5 is internally cooled with circulating water or other conventional cooling fluids to extract heat through the chill surface 3 so as to solidify molten metal 7 exiting the tundish 1. A rotary brush 18 contacts the chill surface 3 to remove debris and other impurities prior to contact with the molten metal.
The casting wheel 5 is supported by journal bearings 9 for rotation about a fixed horizontal axis. The journal bearings 9 are supported on the supporting frame 11 which supports both the bearings and the tundish 1. Although not shown, the bearings and tundish may be supported by separate framework. The casting wheel may be driven by a suitable drive means such as a variable speed motor and reduction gear mechanism, not shown, and a drive chain or belt 13 engaging the casting wheel 5.
After the molten metal 7 contacts the chill surface 3 and is solidified as a strip 15, a coiling apparatus 17 accumulates the strip in coil form for further processing. The cooling apparatus may include roller 23 to guide the cast strip to the coiler.
The continuous casting apparatus 10 may also include a burner 19 to selectively apply heat to the chill surface at a location beneath the tundish 1. In addition, a top roll 21 may be provided which is uncooled or heated, the top roll being mounted for rotation in contact with the molten metal prior to complete solidification of the strip.
In order to properly coil the as-cast strip or further work the cast strip made by the apparatus of FIG. 1 into a product having a satisfactory quality, it is important to provide a cast strip leaving the casting surface having a proper shape or cross-sectional profile.
Difficulties have been encountered in prior art processes in achieving acceptable cast strip cross-section profiles in drag casting of aluminum products. Delivering molten aluminum from a tundish onto a moving chill roller surface produces a sheet product having an increased thickness at the edges thereof. This increased thickness is a result of a faster cooling rate at the edges of the chill surface and a corresponding "dog-bone" effect, or washboard or wavy edge. This condition prevents effective coiling of the cast strip and causes difficulties in further reducing the cast strip in subsequent rolling operations. Cold rolling of sheet or strip product generally requires that the sheet or strip have a slightly thicker center portion than edge portion. Strip having a "dog-bone" shape generally has thick edge portions and a thinner center section.
Besides the problems of inconsistent gauge and shape control, the prior art drag casting apparatus are not adapted to continuously cast metal strip having varying strip widths. In order to change widths, the casting operation must be halted so that a different width tundish may be utilized during the subsequent casting operation.
Accordingly, a need has developed to provide improvements in drag casting of molten metals such as aluminum alloys to continuously cast strips of different widths, wherein the cast strip has acceptable gauge and shape profiles.
In the prior art, it is well known to mechanically slit certain gauges of strip product to remove off-gauge edge material, edge cracking or surface imperfections prior to further processing or shipping to a customer. However, in melt drag casting of aluminum or aluminum alloys, the coiled cast strip is wound at temperatures around 400.degree.-1000.degree. F. These high temperatures restrict the use of mechanical devices contacting the cast strip.
Also in the prior art, and in the field of rapid solidification processing, it is known to continuously cast amorphous type materials while modifying the casting surface to modify the shape of the cast product.
In U.S. Pat. No. 4,197,146 to Frischmann, oblate spheroid segments of an amorphous magnetic material are produced by a rapid solidification process. During the rapid solidification, india ink is applied to a chill roll to define the desired shape and size of the particles to be cast. The alloy melt that makes contact with the ink cools more slowly and causes the metal to separate in the desired form and shape.
U.S. Pat. No. 4,343,347 to Liebermann et al. discloses another rapid solidification method forming a metallic ribbon having cutout patterns therein. A plurality of lines which define the geometric configuration of the cutout are made on the casting surface using a sharp-edged tool or silk screening ink. The line provide a differential cooling rate between the molten metal cast on the lines and the metal cast on the unaltered casting surface.
U.S. Pat. No. 4,155,397 to Honsinger et al. discloses another rapid solidification process forming an amorphous metal alloy. In this patent, the chill roll includes a thermal insulator insert which provides the differential cooling rate to permit casting the patterned amorphous ribbon.
U.S. Pat. No. 4,650,618 to Heinemann et al. discloses an apparatus for producing amorphous strip utilizing projecting areas or recesses in the casting surface to cast a predetermined shape. The projecting areas may be configured as ribs running longitudinally or transversely on the casting surface. The recesses may be circular or randomly shaped.
U.S. Pat. No. 4,982,780 to Stepanenko et al. relates to another continuous casting apparatus, in particular, for producing amorphous metal filaments. In this apparatus, metal filaments having varying widths are produced using a plurality of chilled surfaces rotating in different directions or at different speeds.
U.S. Pat. No. 4,408,653 to Nienart et al. also discloses the use of grooves on a chill surface to produce a particularly shaped continuously cast ribbon or shard.
In the field of twin belt continuous casting, International Publication Number WO 91/12910 to Hugens produces discrete molded shapes by modifying the surface of the casting belts. Japanese Patent Number 63-56337 to Tsuchida casts narrow-width metallic sheet using a cooling roll having spaced apart slit grooves to form the narrow-width sheets.
U.S. Pat. No. 4,527,613 to Bedell et al. discloses another conventional casting apparatus which utilizes a gaseous media to produce a continuous slit line along the cast ribbon length.
However, none of the prior art discussed above addresses the problem of excessive gauge variation on cast strip edges or difficulties in changing cast strip width during melt drag casting of aluminum or aluminum alloys.
In response to this need, the present invention provides a method and apparatus for continuously trimming edge scrap from continuously melt drag cast metal strip. Moreover, the inventive method and apparatus permit adjusting the cast strip width while maintaining continuity of the casting operation.